Define negative externalities
Costs to third parties who were not involved in the transaction between to agents
DWL
There is DWL from producing at the private optimum, instead of pq
Negative externalities in consumption =
Demerit goods
Define positive externalities
Benefits to third parties who were not directly involved in the transaction between the two agents
Positive externalities in consumption =
Merit goods
To move to social optimum for externalities in production
Could use taxation or subsidies.